Error recovery in null data packet (NDP) ranging

ABSTRACT

A first communication device transmits a first physical layer protocol data units (PPDU) that includes a first null data packet announcement (NDPA) frame as part of a first ranging measurement exchange. The first communication device transmits a first null data packet (NDP) as part of the first ranging measurement exchange, and records a transmit time of the first NDP. The first communication device determines whether a second NDP was received correctly from a second communication device as part of the first ranging measurement exchange. In response to determining that the second NDP was not received correctly, the first communication device commences a second ranging measurement exchange, including transmitting a second PPDU that includes a second NDPA frame as part of the second ranging measurement exchange.

This application is a continuation application of U.S. patentapplication entitled “Error Recovery in Null Data packet (NDP) Ranging”,having a Ser. No. 16/179,477, having a filing date of Nov. 2, 2018;which claims the benefit of the U.S. provisional application entitled“CLEAR CHANNEL ASSESSMENT (CCA) AND ERROR RECOVERY OF NULL DATA PACKET(NDP) RANGING”, having a Ser. No. 62/723,946, and having a filling dateof Aug. 28, 2018; which claims the benefit of the U.S. provisionalapplication entitled “CLEAR CHANNEL ASSESSMENT (CCA) AND ERROR RECOVERYOF NULL DATA PACKET (NDP) RANGING”, having a Ser. No. 62/623,419, andhaving a filling date of Jan. 29, 2018, having common inventors, andhaving a common assignee, all of which is incorporated by reference inits entirety.

CROSS REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/623,419, entitled “Clear Channel Assessment (CCA) andError Recovery of Null Data Packet (NDP) Ranging.” filed on Jan. 29,2018, and U.S. Provisional Patent Application No. 62/723,946, entitled“Clear Channel Assessment (CCA) and Error Recovery of Null Data Packet(NDP) Ranging,” filed on Aug. 28, 2018. The applications listed aboveare expressly incorporated herein by reference in their entireties.

FIELD OF TECHNOLOGY

The present disclosure relates generally to wireless communicationsystems, and more particularly to communication exchanges betweenwireless communication devices for ranging measurements among thewireless communication devices.

BACKGROUND

Wireless local area networks (WLANs) have evolved rapidly over the pastdecade, and development of WLAN standards such as the Institute forElectrical and Electronics Engineers (IEEE) 802.11 Standard family hasimproved single-user peak data throughput. For example, the IEEE 802.11bStandard specifies a single-user peak throughput of 11 megabits persecond (Mbps), the IEEE 802.11a and 802.11g Standards specify asingle-user peak throughput of 54 Mbps, the IEEE 802.11n Standardspecifies a single-user peak throughput of 600 Mbps, and the IEEE802.11ac Standard specifies a single-user peak throughput in thegigabits per second (Gbps) range. Future standards promise to provideeven greater throughput, such as throughputs in the tens of Gbps range.

Some mobile communication devices include a WLAN network interface andsatellite positioning technology, such as global positioning system(GPS) technology. GPS technology in mobile communication devices isuseful for navigating to a desired location, for example. However, GPStechnology does not typically provide accurate location information whena GPS receiver is not in direct sight of a GPS satellite, and thus GPStechnology is often not useful for providing location information whilea mobile communication device is within a building such as an airport, ashopping mall, etc., within a tunnel, etc.

Techniques for determining a position of a communication device usingWLAN technology are now under development. For example, a distancebetween a first communication and a second communication device isdetermined by measuring a time of flight of WLAN transmissions betweenthe first communication device and the second communication device, andthe distance is determined based on the measured time of flight.Similarly, distances between the first communication device and multiplethird communication devices are determined. Then, the determineddistances are used to estimate a location of the first communicationdevice by employing, for example, a triangulation technique. For a firstcommunication device having multiple antennas, an angle of departure(AoD) of a WLAN transmission can be determined. Similarly, for a secondcommunication device having multiple antennas, an angle of arrival (AoA)of the WLAN transmission from the first communication device can bedetermined. The AoD and the AoA, along with the determined distances,can be also be used for estimating the location of the firstcommunication device.

SUMMARY

In an embodiment, a method includes: transmitting, by a firstcommunication device, a first physical layer protocol data units (PPDU)that includes a first null data packet announcement (NDPA) frame as partof a first ranging measurement exchange, wherein the first rangingmeasurement exchange is among a set of ranging measurement exchanges;transmitting, by the first communication device, a first null datapacket (NDP) as part of the first ranging measurement exchange;recording, by the first communication device, a transmit time of thefirst NDP; determining, by the first communication device, whether asecond NDP was received correctly from a second communication device aspart of the first ranging measurement exchange; and in response todetermining that the second NDP was not received correctly, commencing,by the first communication device, a second ranging measurementexchange, including transmitting a second PPDU that includes a secondNDPA frame as part of the second ranging measurement exchange, whereinthe second ranging measurement exchange is among the set of rangingmeasurement exchanges.

In another embodiment, an apparatus comprises: a network interfacedevice associated with a first communication device, wherein the networkinterface device comprises one or more integrated circuit (IC) devices.The one or more IC devices are configured to: transmit a first physicallayer protocol data units (PPDU) that includes a first null data packetannouncement (NDPA) frame as part of a first ranging measurementexchange, wherein the first ranging measurement exchange is among a setof ranging measurement exchanges; transmit a first null data packet(NDP) as part of the first ranging measurement exchange; record atransmit time of the first NDP; determine whether a second NDP wasreceived correctly from a second communication device as part of thefirst ranging measurement exchange; and in response to determining thatthe second NDP was not received correctly, commence a second rangingmeasurement exchange, including transmitting a second PPDU that includesa second NDPA frame as part of the second ranging measurement exchange,wherein the second ranging measurement exchange is among the set ofranging measurement exchanges.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example wireless local area network(WLAN), according to an embodiment.

FIG. 2A is a diagram of an example multi-user (MU) ranging measurementexchange in an MU ranging measurement procedure, according to anembodiment.

FIG. 2B is a timing diagram of the example MU ranging measurementexchange of FIG. 2A, according to an embodiment.

FIG. 3A is a diagram of another example MU ranging measurement exchangein an MU ranging measurement procedure, according to another embodiment.

FIG. 3B is a diagram of another example MU ranging measurement exchangein an MU ranging measurement procedure, according to another embodiment.

FIG. 4 is a flow diagram of an example method for performing a rangingmeasurement procedure, according to an embodiment.

FIG. 5 is a flow diagram of another example method for performing aranging measurement procedure, according to an embodiment.

FIG. 6 is a diagram of a set of example single-user (SU) rangingmeasurement exchanges in an SU ranging measurement procedure, accordingto an embodiment.

FIG. 7 is a flow diagram of an example method performed by acommunication device in connection with an SU ranging measurementexchange, according to an embodiment.

FIG. 8 is a flow diagram of another example method performed by acommunication device in connection with an SU ranging measurementexchange, according to another embodiment.

FIG. 9 is a flow diagram of another example method performed by acommunication device in connection with an SU ranging measurementexchange, according to another embodiment.

FIG. 10 is a flow diagram of an example method for performing rangingmeasurement exchanges, according to an embodiment.

DETAILED DESCRIPTION

Ranging measurement procedures and techniques described below arediscussed in the context of wireless local area networks (WLANs) thatutilize protocols the same as or similar to protocols defined by the802.11 Standard from the Institute of Electrical and ElectronicsEngineers (IEEE) merely for explanatory purposes. In other embodiments,however, ranging measurement procedures and techniques are utilized inother types of wireless communication systems such as personal areanetworks (PANs), mobile communication networks such as cellularnetworks, metropolitan area networks (MANs), etc.

FIG. 1 is a block diagram of an example WLAN 110, according to anembodiment. The WLAN 110 includes an access point (AP) 114 thatcomprises a host processor 118 coupled to a network interface device122. The network interface 122 includes a medium access control (MAC)processor 126 and a physical layer (PHY) processor 130. The PHYprocessor 130 includes a plurality of transceivers 134, and thetransceivers 134 are coupled to a plurality of antennas 138. Althoughthree transceivers 134 and three antennas 138 are illustrated in FIG. 1,the AP 114 includes other suitable numbers (e.g., 1, 2, 4, 5, etc.) oftransceivers 134 and antennas 138 in other embodiments. In someembodiments, the AP 114 includes a higher number of antennas 138 thantransceivers 134, and antenna switching techniques are utilized.

The network interface 122 is implemented using one or more integratecircuits (ICs) configured to operate as discussed below. For example,the MAC processor 126 may be implemented, at least partially, on a firstIC, and the PHY processor 130 may be implemented, at least partially, ona second IC. As another example, at least a portion of the MAC processor126 and at least a portion of the PHY processor 130 may be implementedon a single IC. For instance, the network interface 122 may beimplemented using a system on a chip (SoC), where the SoC includes atleast a portion of the MAC processor 126 and at least a portion of thePHY processor 130.

In an embodiment, the host processor 118 includes a processor configuredto execute machine readable instructions stored in a memory device (notshown) such as a random access memory (RAM), a read-only memory (ROM), aflash memory, etc. In an embodiment, the host processor 118 may beimplemented, at least partially, on a first IC, and the network device122 may be implemented, at least partially, on a second IC. As anotherexample, the host processor 118 and at least a portion of the networkinterface 122 may be implemented on a single IC.

In various embodiments, the MAC processor 126 and/or the PHY processor130 of the AP 114 are configured to generate data units, and processreceived data units, that conform to a WLAN communication protocol suchas a communication protocol conforming to the IEEE 802.11 Standard oranother suitable wireless communication protocol. For example, the MACprocessor 126 may be configured to implement MAC layer functions,including MAC layer functions of the WLAN communication protocol, andthe PHY processor 130 may be configured to implement PHY functions,including PHY functions of the WLAN communication protocol. Forinstance, the MAC processor 126 may be configured to generate MAC layerdata units such as MAC service data units (MSDUs), MAC protocol dataunits (MPDUs), etc., and provide the MAC layer data units to the PHYprocessor 130. The PHY processor 130 may be configured to receive MAClayer data units from the MAC processor 126 and encapsulate the MAClayer data units to generate PHY data units such as PHY protocol dataunits (PPDUs) for transmission via the antennas 138. Similarly, the PHYprocessor 130 may be configured to receive PHY data units that werereceived via the antennas 138, and extract MAC layer data unitsencapsulated within the PHY data units. The PHY processor 130 mayprovide the extracted MAC layer data units to the MAC processor 126,which processes the MAC layer data units.

The PHY processor 130 is configured to downconvert one or more radiofrequency (RF) signals received via the one or more antennas 138 to oneor more baseband analog signals, and convert the analog basebandsignal(s) to one or more digital baseband signals, according to anembodiment. The PHY processor 130 is further configured to process theone or more digital baseband signals to demodulate the one or moredigital baseband signals and to generate a PPDU. The PHY processor 130includes amplifiers (e.g., a low noise amplifier (LNA), a poweramplifier, etc.), a radio frequency (RF) downconverter, an RFupconverter, a plurality of filters, one or more analog-to-digitalconverters (ADCs), one or more digital-to-analog converters (DACs), oneor more discrete Fourier transform (DFT) calculators (e.g., a fastFourier transform (FFT) calculator), one or more inverse discreteFourier transform (IDFT) calculators (e.g., an inverse fast Fouriertransform (IFFT) calculator), one or more modulators, one or moredemodulators, etc.

The PHY processor 130 is configured to generate one or more RF signalsthat are provided to the one or more antennas 138. The PHY processor 130is also configured to receive one or more RF signals from the one ormore antennas 138.

The MAC processor 126 is configured to control the PHY processor 130 togenerate one or more RF signals by, for example, providing one or moreMAC layer data units (e.g., MPDUs) to the PHY processor 130, andoptionally providing one or more control signals to the PHY processor130, according to some embodiments. In an embodiment, the MAC processor126 includes a processor configured to execute machine readableinstructions stored in a memory device (not shown) such as a RAM, a readROM, a flash memory, etc. In an embodiment, the MAC processor 126includes a hardware state machine.

The WLAN 110 includes a plurality of client stations 154. Although threeclient stations 154 are illustrated in FIG. 1, the WLAN 110 includesother suitable numbers (e.g., 1, 2, 4, 5, 6, etc.) of client stations154 in various embodiments. The client station 154-1 includes a hostprocessor 158 coupled to a network interface device 162. The networkinterface 162 includes a MAC processor 166 and a PHY processor 170. ThePHY processor 170 includes a plurality of transceivers 174, and thetransceivers 174 are coupled to a plurality of antennas 178. Althoughthree transceivers 174 and three antennas 178 are illustrated in FIG. 1,the client station 154-1 includes other suitable numbers (e.g., 1, 2, 4,5, etc.) of transceivers 174 and antennas 178 in other embodiments. Insome embodiments, the client station 154-1 includes a higher number ofantennas 178 than transceivers 174, and antenna switching techniques areutilized.

The network interface 162 is implemented using one or more ICsconfigured to operate as discussed below. For example, the MAC processor166 may be implemented on at least a first IC, and the PHY processor 170may be implemented on at least a second IC. As another example, at leasta portion of the MAC processor 166 and at least a portion of the PHYprocessor 170 may be implemented on a single IC. For instance, thenetwork interface 162 may be implemented using an SoC, where the SoCincludes at least a portion of the MAC processor 166 and at least aportion of the PHY processor 170.

In an embodiment, the host processor 158 includes a processor configuredto execute machine readable instructions stored in a memory device (notshown) such as a RAM, a ROM, a flash memory, etc. In an embodiment, thehost processor 158 may be implemented, at least partially, on a firstIC, and the network device 162 may be implemented, at least partially,on a second IC. As another example, the host processor 158 and at leasta portion of the network interface 162 may be implemented on a singleIC.

In various embodiments, the MAC processor 166 and the PHY processor 170of the client device 154-1 are configured to generate data units, andprocess received data units, that conform to the WLAN communicationprotocol or another suitable communication protocol. For example, theMAC processor 166 may be configured to implement MAC layer functions,including MAC layer functions of the WLAN communication protocol, andthe PHY processor 170 may be configured to implement PHY functions,including PHY functions of the WLAN communication protocol. The MACprocessor 166 may be configured to generate MAC layer data units such asMSDUs, MPDUs, etc., and provide the MAC layer data units to the PHYprocessor 170. The PHY processor 170 may be configured to receive MAClayer data units from the MAC processor 166 and encapsulate the MAClayer data units to generate PHY data units such as PPDUs fortransmission via the antennas 178. Similarly, the PHY processor 170 maybe configured to receive PHY data units that were received via theantennas 178, and extract MAC layer data units encapsulated within thePHY data units. The PHY processor 170 may provide the extracted MAClayer data units to the MAC processor 166, which processes the MAC layerdata units.

The PHY processor 170 is configured to downconvert one or more RFsignals received via the one or more antennas 178 to one or morebaseband analog signals, and convert the analog baseband signal(s) toone or more digital baseband signals, according to an embodiment. ThePHY processor 170 is further configured to process the one or moredigital baseband signals to demodulate the one or more digital basebandsignals and to generate a PPDU. The PHY processor 170 includesamplifiers (e.g., an LNA, a power amplifier, etc.), an RF downconverter,an RF upconverter, a plurality of filters, one or more ADCs, one or moreDACs, one or more DFT calculators (e.g., an FFT calculator), one or moreIDFT calculators (e.g., an IFFT calculator), one or more modulators, oneor more demodulators, etc.

The PHY processor 170 is configured to generate one or more RF signalsthat are provided to the one or more antennas 178. The PHY processor 170is also configured to receive one or more RF signals from the one ormore antennas 178.

The MAC processor 166 is configured to control the PHY processor 170 togenerate one or more RF signals by, for example, providing one or moreMAC layer data units (e.g., MPDUs) to the PHY processor 170, andoptionally providing one or more control signals to the PHY processor170, according to some embodiments. In an embodiment, the MAC processor166 includes a processor configured to execute machine readableinstructions stored in a memory device (not shown) such as a RAM, a ROM,a flash memory, etc. In an embodiment, the MAC processor 166 includes ahardware state machine.

In an embodiment, each of the client stations 154-2 and 154-3 has astructure that is the same as or similar to the client station 154-1.Each of the client stations 154-2 and 154-3 has the same or a differentnumber of transceivers and antennas. For example, the client station154-2 and/or the client station 154-3 each have only two transceiversand two antennas (not shown), according to an embodiment.

PPDUs are sometimes referred to herein as packets. MPDUs are sometimesreferred to herein as frames.

FIG. 2A is a diagram of an example multi-user (MU) ranging measurementexchange 200 in an MU ranging measurement procedure, according to anembodiment. The diagram 200 is described in the context of the examplenetwork 110 merely for explanatory purposes. In some embodiments,signals illustrated in FIG. 2A are generated by other suitablecommunication devices in other suitable types of wireless networks.

The MU ranging measurement exchange 200 corresponds to an AP-initiatedMU ranging measurement exchange, according to an embodiment. The MUranging measurement exchange 200 includes an uplink (UL) null datapacket (NDP) portion 204, a downlink (DL) NDP portion 208, a DL feedback(FB) portion 210, and an UL FB portion 212. In an embodiment, the uplinkUL NDP portion 204, the DL NDP portion 208, the DL FB portion 210, andthe UL FB portion 212 occur within a single transmit opportunity period(TXOP). In another embodiment, the uplink UL NDP portion 204, the DL NDPportion 208, the DL FB portion 210, and the UL FB portion 212 do notoccur within a single TXOP. For example, the uplink UL NDP portion 204and the DL NDP portion 208 occur within a single TXOP, whereas the DL FBportion 210 and the UL FB portion 212 occur after the single TXOP (e.g.,in another TXOP or in multiple other TXOPs).

In the UL NDP portion 204, a first communication device (e.g., the AP114) transmits a DL PPDU 216 that includes a trigger frame to cause agroup of multiple second communication devices (e.g., client stations154) to simultaneously transmit, as part of an uplink (UL) MUtransmission 220, UL null data packets (NDPs) 224. In an embodiment, thetrigger frame in the PPDU 216 is a type of trigger frame specificallyfor initiating an MU ranging measurement exchange such as the MU rangingmeasurement exchange 200. The trigger frame in the PPDU 216 causesmultiple client stations 154 to begin simultaneously transmitting the ULMU transmission 220 a defined time period after an end of the PPDU 216.In an embodiment, the defined time period is a short interframe space(SIFS) as defined by the IEEE 802.11 Standard. In other embodiments,another suitable time period is utilized.

In an embodiment, the UL MU transmission 220 includes an UL MU multipleinput, multiple output (MIMO) transmission having two or more UL NDPs224 from multiple client stations 154, e.g., STA1, STA2, STA3, and STA4.The two or more of the UL NDPs 224 are transmitted within a samefrequency band via different spatial streams (e.g., MU-MIMO). In anotherembodiment, the UL MU transmission 220 includes an UL orthogonalfrequency division multiple access (OFDMA) transmission having two ormore UL NDPs 224 from multiple client stations 154, e.g., STA1, STA2,STA3, and STA4, in different respective frequency bandwidth portions. Inyet another embodiment, three or more UL NDP packets 224 transmittedusing a combination of UL MU-MIMO and UL OFDMA, where at least two NDPsare transmitted using MU-MIMO in a same frequency bandwidth portion viadifferent spatial streams, and at least one NDP is transmitted in atleast one other different frequency bandwidth portion. The UL NDPs 224include PHY preambles having one or more short training fields (STFs),one or more long training fields (LTFs) and one or more signal fields,in an embodiment. In an embodiment, each PHY preamble of each UL NDP 224includes i) a legacy portion having a legacy STF (L-STF), a legacy LTF(L-LTF), and a legacy signal field (L-SIG), and ii) a non-legacy portionhaving one or more non-legacy training fields (e.g., a non-legacy LTF(NL-LTF)) and one or more non-legacy signal fields (NL-SIGs). The ULNDPs 224 omit PHY data portions.

When transmitting the UL NDPs 224, each client station 154 records atime t_(1,k) at which the client station 154 began transmitting aparticular portion of the UL NDP 224 (e.g., a first occurring NL-LTF inthe UL NDP 224), where k is an index indicating the particular clientstation 154. A time such as t_(1,k) is sometimes referred to herein as a“transmit time” or a “time of departure” (ToD).” Similarly, when the AP114 receives each UL NDP 224, the AP 114 records a time t_(2,k) at whichthe AP 114 began receiving the particular portion of the UL NDP 224(e.g., the first occurring NL-LTF in the UL NDP 224). A time such ast_(2,k) is sometimes referred to herein as a “receive time” or a “timeof arrival” (ToD).”

In some embodiments, when transmitting the UL NDPs 224, each of at leastsome of the client stations 154 (e.g., client stations 154 with multipleantennas 174) records an angle of departure. AoD_(1,k), at which the ULNDP 224 left the antennas 178 of the client station 154. Similarly, whenthe AP 114 receives each UL NDP 224, the AP 114 records an angle ofarrival, AoA_(1,k), at which the UL NDP 224 arrived at the antennas 138of the AP 114.

FIG. 2B is a timing diagram of the example MU ranging measurementexchange 200 of FIG. 2A. As illustrated in FIG. 2B, each client station154 records the time t_(1,k) at which the client station 154 begantransmitting the particular portion of the UL NDP 224 (e.g., the firstoccurring NL-LTF in the UL NDP 224), and records the AoD_(1,k) at whichthe UL NDP 224 left the antennas 178 of the client station 154.Additionally, the AP 114 records the time t_(2,k) at which the AP 114began receiving the particular portion (e.g., a first occurring NL-LTF)in each UL NDP 224, and the AoA_(1,k), at which each UL NDP 224 arrivedat the antennas 138 of the AP 114.

Referring now to FIGS. 2A and 2B, responsive to the UL MU transmission220, the AP 114 begins transmitting a DL PPDU 228 that includes an NDPannouncement (NDPA) frame a defined time period after an end of the ULMU transmission 220. In an embodiment, the defined time period is SIFS.In other embodiments, another suitable time period is utilized. The NDPAframe in the PPDU 228 is configured to cause the client stations 154 tobe prepared to receive an NDP from the AP 114, according to anembodiment.

The AP 114 generates a DL PPDU 232 and begins transmitting the DL PPDU232 a defined time period after an end of the DL PPDU 228. In anembodiment, the defined time period is SIFS. In other embodiments,another suitable time period is utilized. The DL PPDU 232 is a MU PPDUthat includes DL NDPs 236 to respective client stations 154. In anotherembodiment, the AP 114 transmits a single DL NDP 236 using a SU DLtransmission (e.g., with a broadcast destination address or a multicastdestination address) to the client stations 154. The DL NDP(s) 236include PHY preamble(s) having one or more STFs, one or more LTFs andone or more signal fields, in an embodiment. In an embodiment, the PHYpreamble of the DL NDP 236 includes i) a legacy portion having an L-STF,an L-LTF, and an L-SIG, and ii) a non-legacy portion having one or moreNL-LTFs, and one or more NL-SIGs. The DL NDP(s) 236 omit PHY dataportions. In an embodiment, different DL NDPs 236 are transmitted indifferent frequency bandwidth portions (e.g., OFDMA). In someembodiments, two or more of the DL NDPs 236 are transmitted within asame frequency band (e.g., two or more of the DL NDPs 236 span the samefrequency band) using different spatial streams (e.g., the two or moreDL NDPs 236 are transmitted using MU-MIMO). In another embodiment, asingle DL NDP 236 is broadcast or multicast to the client stations 154.

When transmitting the DL NDP(s) 236, the AP 114 records a time t_(3,k)at which the AP 114 began transmitting a particular portion of the DLNDP(s) 236 (e.g., a first occurring NL-LTF in each DL NDP(s) 236).Similarly, when each client station 154 receives the corresponding DLNDP 236, the client station 154 records a time t_(4,k) at which theclient station 154 began receiving the particular portion of the DL NDP236 (e.g., the first occurring NL-LTF in the DL NDP 236). As illustratedin FIG. 2B, the AP 114 records the time t_(3,k) at which the AP 114began transmitting the particular portion of the DL NDP 236 (e.g., thefirst occurring NL-LTF in the DL NDP 236), and the client station 154records the time t_(4,k) at which the client station 154 began receivingthe particular portion of the DL NDP 236 (e.g., the first occurringNL-LTF in the DL NDP 236).

In some embodiments, when transmitting the DL NDP 236, the AP 114records an AoD_(2,k) at which the DL NDP 236 left the antennas 138 ofthe AP 114. Similarly, when the client station 154 receives the DL NDP236, the client station 154 records an AoA_(2,k) at which the DL NDP 236arrived at the antennas 178 of the client station 154.

In some embodiments, the MU ranging measurement exchange 200 omits theDL PPDU 228. For example, the AP 114 begins transmitting the DL PPDU 232a defined time period after an end of the UL MU transmission 220. In anembodiment, the defined time period is SIFS. In other embodiments,another suitable time period is utilized.

The DL FB portion 210 includes a DL PPDU 240 (which may be a DL OFDMAtransmission or a DL MU-MIMO transmission) having FB frames 244 formultiple client stations 154, e.g., STA1, STA2, STA3, and STA4. The FBframes 244 are illustrated in FIG. 2A as being transmitted in differentfrequency bandwidth portions. In some embodiments, two or more of the FBframes 244 are transmitted within a same frequency band (e.g., two ormore of the FB frames 244 span the same frequency band) using differentspatial streams (e.g., the two or more FB frames 244 are transmittedusing MU-MIMO).

In some embodiments, the DL PPDU 240 is transmitted a defined timeperiod after an end of the DL PPDU 232. In an embodiment, the definedtime period is SIFS. In other embodiments, another suitable time periodis utilized. In other embodiments, the DL PPDU 240 is transmitted aftersome delay. As discussed above, in some embodiments, the DL PPDU 240 isnot transmitted within a same TXOP as the DL PPDU 232.

The FB frames 244 respectively include the recorded times t_(2,k) andt_(3,k). In some embodiments, each of one or more FB frames 244respectively includes (optionally) the recorded angles AoA_(1,k) andAoD_(2,k). In some embodiments, the FB frames 244 optionally alsoinclude respective channel estimate information determined by the AP 114based on reception of the UL NDPs 224.

In another embodiment, the FB frames 244 respectively include recordedtimes t_(2,k) and t_(3,k), corresponding to a previous MU rangingmeasurement exchange (similar to the MU ranging measurement exchange200) that occurred prior to the MU ranging measurement exchange 200. Insome embodiments, each of one or more FB frames 244 respectivelyincludes (optionally) the recorded angles AoA_(1,k) and AoD_(2,k)corresponding to the previous MU ranging measurement exchange thatoccurred prior to the MU ranging measurement exchange 200. In someembodiments, the FB frames 244 optionally also include respectivechannel estimate information determined by the AP 114 based on receptionof UL NDPs corresponding to the previous MU ranging measurement exchangethat occurred prior to the MU ranging measurement exchange 200.

After receipt of the FB frames 244, one or more of the client stations154 respectively calculate one or more respective of times-of-flightbetween the AP 114 and the one or more client stations 154 using therecorded times t_(1,k), t_(2,k), t_(3,k), and t_(4,k), according to anembodiment. Any suitable technique, including currently knowntechniques, may be utilized to calculate a time-of-flight using therecorded times t_(1,k), t_(2,k), t_(3,k), and t_(4,k). Respectivedistances between the AP 114 and the client stations 154 may becalculated using the calculated times-of-flight, e.g., by respectivelymultiplying the times-of-flight by the speed of light, according to anembodiment.

In another embodiment, in which the FB frames 244 respectively includerecorded times t_(2,k) and t_(3,k) corresponding to the previous MUranging measurement exchange that occurred prior to the MU rangingmeasurement exchange 200, one or more of the client stations 154respectively calculate one or more respective of times-of-flight betweenthe AP 114 and the one or more client stations 154 using the recordedtimes t_(1,k), t_(2,k), t_(3,k), and t_(4,k) that correspond to theprevious MU ranging measurement exchange, according to an embodiment.

In some embodiments, one or more of the client stations 154 calculatesestimated positions of one or more of the client stations using thecalculated times-of-flight. For example, the client station 154-1 usestriangulation techniques to calculate an estimated positions of theclient station 154-1 using the calculated time-of-flight. In someembodiments, the client station 154-1 calculates an estimated positionof the client station also using the recorded angles AoD_(1,k),AoA_(1,k), AoD_(2,k), and AoA_(2,k). For example, the recorded anglesAoD_(1,k), AoA_(1,k), AoD_(2,k), and AoA_(2,k) are used as part of atriangulation algorithm for determining a position of the client station154-1.

Responsive to receipt of the FB frames 244, the client stations 154generate an UL MU transmission 250 (which may be an UL OFDMAtransmission or an UL MU MIMO transmission) that includes respective ACKframes 254 from respective client stations, according to an embodiment.The client stations 154 transmit as part of the UL MU transmission 250 adefined time period after an end of the DL transmission 240. In anembodiment, the defined time period is SIFS. In other embodiments,another suitable time period is utilized. The ACK frames 254 areillustrated in FIG. 2A as being transmitted in different frequencybandwidth portions. In some embodiments, two or more of the ACK frames254 are transmitted within a same frequency band (e.g., two or more ofthe ACK frames 254 span the same frequency band) using different spatialstreams (e.g., the two or more ACK frames 254 are transmitted usingMU-MIMO). In some embodiments, the client stations 154 do not generateand transmit the UL MU transmission 250 (e.g., the client stations 154do not generate and transmit the AC frames 254).

In an embodiment in which client stations 154 are to transmit the ULACKs 254 in response to correctly receiving the DL FB frames 244, andwhen the AP 114 does not receive a UL ACK(s) 254 from one or more of theclient stations 154, the AP 114 retransmits one or more of the FB frames244 to the client station(s) 154 that did not transmit the UL ACK(s)254. In an embodiment, the retransmission the FB frames 244 is includedin a DL PPDU (not shown) that is transmitted after a time period bywhich the AP 114 expected to receive the UL ACK 254.

In an embodiment, the AP 114 transmits a DL PPDU 260 a defined timeperiod after an end of the UL MU transmission 250. In an embodiment, thedefined time period is SIFS. In other embodiments, another suitable timeperiod is utilized. The PPDU 260 includes a trigger frame to cause thegroup of client stations 154 to simultaneously transmit, as part of anUL MU transmission 264, uplink PPDUs 268 that include rangingmeasurement feedback. The trigger frame in the PPDU 260 causes multipleclient stations 154 to begin simultaneously transmitting the UL MUtransmission 264 a defined time period after an end of the PPDU 260. Inan embodiment, the defined time period is SIFS. In other embodiments,another suitable time period is utilized.

The UL MU transmission 264 (which may be an UL OFDMA transmission or anUL MU-MIMO transmission) includes UL PPDUs 268 from multiple clientstations 154, e.g., STA1, STA2, STA3, and STA4. The UL PPDUs 268 areillustrated in FIG. 2A as being transmitted in different frequencybandwidth portions. In some embodiments, two or more of the UL PPDUs 268are transmitted within a same frequency band (e.g., two or more of theUL PPDUs 268 span the same frequency band) using different spatialstreams (e.g., the two or more UL PPDUs 268 are transmitted usingMU-MIMO).

The UL PPDUs 268 correspond to uplink ranging measurement feedbackpackets. The PPDUs 268 respectively include the recorded times t_(1,k)and t_(4,k). In some embodiments, each of one or more PPDUs 268respectively includes (optionally) the recorded angles AoD_(1,k) andAoA_(2,k). In some embodiments, the PPDUs 268 optionally also includerespective channel estimate information determined by the client station154 based on reception of the DL NDP 236.

In another embodiment in which a previous MU ranging measurementexchange (similar to the MU ranging measurement exchange 200) occurredprior to the MU ranging measurement exchange 200, the UL PPDUs 268respectively include recorded times t_(1,k) and t_(4,k), correspondingto the previous MU ranging measurement exchange. In some embodiments,each of one or more FB frames 244 respectively includes (optionally) therecorded angles AoD_(1,k) and AoA_(2,k) corresponding to the previous MUranging measurement exchange that occurred prior to the MU rangingmeasurement exchange 200. In some embodiments, the UL PPDUs 268optionally also include respective channel estimate informationdetermined by the AP 114 based on reception of UL NDPs corresponding tothe previous MU ranging measurement exchange that occurred prior to theMU ranging measurement exchange 200.

After receipt of the UL PPDUs 268, the AP 114 calculates respective oftimes-of-flight between the AP 114 and the client stations 154 using therecorded times t_(1,k), t_(2,k), t_(3,k), and t_(4,k), according to anembodiment. Any suitable technique, including currently knowntechniques, may be utilized to calculate a time-of-flight using therecorded times t_(1,k), t_(2,k), t_(3,k), and t_(4,k). Respectivedistances between the AP 114 and the client stations 154 may becalculated using the calculated times-of-flight, e.g., by respectivelymultiplying the times-of-flight by the speed of light, according to anembodiment.

In another embodiment in which the UL PPDUs 268 respectively includerecorded times t_(1,k) and t_(4,k) corresponding to the previous MUranging measurement exchange that occurred prior to the MU rangingmeasurement exchange 200, the AP 114 calculates one or more respectiveof times-of-flight between the AP 114 and the one or more clientstations 154 using the recorded times t_(1,k), t_(2,k), t_(3,k), andt_(4,k) that correspond to the previous MU ranging measurement exchange,according to an embodiment.

In some embodiments, the AP 114 calculates estimated positions of one ormore of the client stations using the calculated times-of-flight. Forexample, the AP 114 uses triangulation techniques to calculate estimatedpositions of one or more of the client stations using the calculatedtimes-of-flight. In some embodiments, the AP 114 calculates estimatedpositions of one or more of the client stations also using the recordedangles AoD_(1,k), AoA_(1,k), AoD_(2,k), and AoA_(2,k). For example, therecorded angles AoD_(1,k), AoA_(1,k), AoD_(2,k), and AoA_(2,k) are usedas part of a triangulation algorithm for determining positions ofcommunication devices.

In an embodiment, responsive to receipt of the FB frames 268, the AP 114generates a DL transmission (not shown in FIG. 1) that includes ACKinformation that indicates the AP 114 correctly received the FB frames268. The AP 114 transmits the DL transmission (not shown in FIG. 1) thatincludes the ACK information a defined time period after an end of theUL transmission 264. In an embodiment, the defined time period is SIFS.In other embodiments, another suitable time period is utilized. In someembodiments, the AP 114 does not generate and transmit a DL transmissionthat includes ACK information that indicates the AP 114 correctlyreceived the FB frames 268.

In another embodiment, the order, in time, of the DL FB portion 210 andthe UL FB portion 212 is reversed, and the UL FB portion 212 occursbefore the DL FB portion 210. In some embodiments, the DL FB portion 210is omitted. In some embodiments, the UL FB portion 212 is omitted.

FIG. 3A is a diagram of another example MU ranging measurement exchange300 in an MU ranging measurement procedure, according to an embodiment.The MU ranging measurement exchange 300 is similar to the MU rangingmeasurement exchange 200 of FIG. 2A, but omits the UL FB portion 212. Inan embodiment, the UL transmission 250 is omitted.

FIG. 3B is a diagram of another example MU ranging measurement exchange350 in an MU ranging measurement procedure, according to an embodiment.The MU ranging measurement exchange 350 is similar to the MU rangingmeasurement exchange 200 of FIG. 2A, but omits the DL FB portion 210.Additionally, an UL FB portion 354 includes a DL transmission 358corresponding to a multi-station block acknowledgment (M-BA) toacknowledge the UL FB frames 268. In an embodiment, the DL transmission358 is omitted.

Referring now to FIGS. 2A, 3A, and 3B, in an embodiment, the triggerframe 216 includes a sub-field that indicates whether the clientstations 154 that are to transmit the UL NDPs 224 are to perform a clearchannel assessment (CCA) operation prior to transmitting as part of theUL transmission 220. In an embodiment, the sub-field is a single bitthat indicates whether the client stations 154 that are to transmit theUL NDPs 224 are to perform the CCA operation prior to transmitting aspart of the UL transmission 220. In an embodiment, the CCA operationincludes a physical CCA, which involves measuring an energy level in acommunication channel and comparing the energy level to a threshold. Inan embodiment, the energy level being below the threshold indicates thatthe communication channel is idle; whereas the energy level meeting thethreshold indicates that the communication channel is busy.

In an embodiment, the CCA operation also includes a virtual CCA, whichinvolves monitoring a timer that indicates whether another communicationdevice is using the communication channel. For example, when the clientstation 154 receives a packet, the client station 154 examines a valueof a duration field in a MAC header of the packet and sets a networkallocation vector (NAV) timer according to the value of the durationfield. The NAV timer then counts down at a predetermined rate. When theNAV counter reaches zero, this indicates that the communication channelis idle, according to an embodiment. In an embodiment, the virtual CCAincludes examining a NAV timer—the NAV timer being zero indicates thatthe communication channel is idle; whereas the NAV timer being non-zeroindicates that the communication channel is busy. In an embodiment, whensetting the NAV timer in response to receiving a packet, the clientstation 154 records a network address (e.g., a MAC address, anassociation identifier (AID), etc.) of the communication device thattransmitted the packet to associate the NAV timer with the communicationdevice that transmitted the packet. In some situations, the clientstation 154 concludes the virtual CCA indicates the channel is idle evenwhen the NAV timer is non-zero for a particular value of the networkaddress associated with the setting of the NAV timer, according to someembodiments.

FIG. 4 is a flow diagram of an example method 400 for performing aranging measurement procedure, according to an embodiment. In someembodiments, the network interface device 122 of FIG. 1 is configured toimplement the method 400. The method 400 is described in the context ofthe network interface device 122 merely for explanatory purposes and, inother embodiments, the method 400 is implemented by another suitabledevice. For instance, in an embodiment, the network interface device 162of FIG. 1, or another suitable WLAN network interface device isconfigured to implement the method 400.

At block 404, the network interface device 122 determines whether clientstations are to perform a CCA operation prior to transmitting UL NDPs(e.g., UL NDPs 224) as part of a ranging measurement exchange (e.g., theranging measurement exchange 200, the ranging measurement exchange 300,the ranging measurement exchange 350, etc.). In an embodiment,determining whether client stations are to perform a CCA operation priorto transmitting UL NDPs as part of a ranging measurement exchange isbased on determining a level of use of a communication channel. Forexample, determining whether client stations are to perform a CCAoperation prior to transmitting UL NDPs as part of a ranging measurementexchange is based on determining how many communication devices are in acommunication network (BSS) in which the network interface device 122 isoperating, according to an embodiment. For instance, if a number ofcommunication devices in the communication network meets a threshold,the network interface device 122 determines that client stations are toperform a CCA operation prior to transmitting UL NDPs as part of aranging measurement exchange, whereas if the number of communicationdevices in the communication network is below the threshold, the networkinterface device 122 determines that client stations are not to performthe CCA operation prior to transmitting UL NDPs as part of a rangingmeasurement exchange, according to an embodiment.

As another example, determining whether client stations are to perform aCCA operation prior to transmitting UL NDPs as part of a rangingmeasurement exchange is based on determining how many othercommunication networks (sometimes referred to as “neighboring networks”or “neighboring BSSs”) are proximate a communication network (BSS) inwhich the network interface device 122 is operating, according to anembodiment. For instance, if a number of neighboring networks meets athreshold, the network interface device 122 determines that clientstations are to perform a CCA operation prior to transmitting UL NDPs aspart of a ranging measurement exchange, whereas if the number ofneighboring networks is below the threshold, the network interfacedevice 122 determines that client stations are not to perform the CCAoperation prior to transmitting UL NDPs as part of a ranging measurementexchange, according to an embodiment.

At block 408, the network interface device 122 generates a trigger frameto prompt a group of client stations 154 to transmit UL NDPs (e.g., ULNDPs 224) as part of a ranging measurement exchange (e.g., the rangingmeasurement exchange 200, the ranging measurement exchange 300, theranging measurement exchange 350, etc.). In an embodiment, the networkinterface device 122 generates the trigger frame to indicate whetherclient stations are to perform a CCA operation prior to transmitting theUL NDPs (e.g., UL NDPs 224) as part of the ranging measurement exchange.In an embodiment, the network interface device 122 generates the triggerframe to include a sub-field that indicates whether client stations areto perform a CCA operation prior to transmitting the UL NDPs (e.g., ULNDPs 224) as part of the ranging measurement exchange. In an embodiment,the network interface device 122 generates the trigger frame to indicatewhether client stations are to perform a CCA operation prior totransmitting the UL NDPs (e.g., UL NDPs 224) as part of the rangingmeasurement exchange in accordance with the determination made at block404.

At block 412, the network interface device 122 transmits the triggerframe to prompt the group of client stations 154 to transmit UL NDPs(e.g., UL NDPs 224) as part of the ranging measurement exchange (e.g.,the ranging measurement exchange 200, the ranging measurement exchange300, the ranging measurement exchange 350, etc.).

FIG. 5 is a flow diagram of an example method 500 for performing aranging measurement procedure, according to an embodiment. In someembodiments, the network interface device 162 of FIG. 1 is configured toimplement the method 500. The method 400 is described in the context ofthe network interface device 162 merely for explanatory purposes and, inother embodiments, the method 500 is implemented by another suitabledevice. For instance, in an embodiment, the network interface device 122of FIG. 1, or another suitable WLAN network interface device isconfigured to implement the method 500.

At block 504, the network interface device 162 receives a trigger frame(e.g., the trigger frame 216) as part of a ranging measurement exchange(e.g., the ranging measurement exchange 200, the ranging measurementexchange 300, the ranging measurement exchange 350, etc.). In anembodiment, the trigger frame is configured to prompt a plurality ofcommunication devices to transmit NDPs as part of an UL MU transmission(the UL MU transmission 220) as part of the ranging measurementexchange.

At block 508, the network interface device 162 determines whether thetrigger frame (received at block 504) indicates that the networkinterface device 162 is to perform a CCA procedure prior to transmittingan NDP in response to the trigger frame. In an embodiment, block 508includes analyzing a sub-field in the trigger frame to determine whetherthe sub-field indicates that the network interface device 162 is toperform a CCA procedure prior to transmitting an NDP in response to thetrigger frame.

In response to determining, at block 508, that the trigger frame(received at block 504) does not indicate that the network interfacedevice 162 is to perform a CCA procedure prior to transmitting an NDP inresponse to the trigger frame, the flow proceeds to block 512. At block512, the network interface device 162 transmits an NDP in response tothe trigger frame received at block 504. In an embodiment, block 512includes transmitting the NDP a defined time period (e.g., SIFS) afteran end of reception of a PPDU that includes the trigger frame (receivedat block 504). In an embodiment and when block 512 is performed inresponse to determining, at block 508, that the trigger frame does notindicate that the network interface device 162 is to perform a CCAprocedure, block 512 is performed without first performing the CCAoperation in connection with transmitting the NDP. In an embodiment andwhen block 512 is performed in response to determining, at block 508,that the trigger frame does not indicate that the network interfacedevice 162 is to perform a CCA procedure, block 512 is performedregardless of whether a communication channel (via which the NDP isbeing transmitted) is busy.

On the other hand, if the network interface device 162 determines, atblock 508, that the trigger frame (received at block 504) indicates thatthe network interface device 162 is to perform the CCA procedure priorto transmitting the NDP, the flow proceeds to block 516. At block 516,the network interface device 162 performs the CCA operation to determinewhether the communication channel is busy. In an embodiment, performingthe CCA operation includes performing one or both of: i) the physicalCCA (discussed above), and ii) the virtual CCA (discussed above). In anembodiment, performing the CCA operation includes performing the CCAoperation after an end of reception of a PPDU that includes the triggerframe (received at block 504). In an embodiment, performing the CCAoperation includes performing at least the physical CCA for a contiguoustime period that occurs prior to a time at which the network interfacedevice 162 is to transmit the NDP in response to the trigger framereceived at block 504. In an embodiment, performing the virtual CCAincludes concluding that the virtual CCA indicates the communicationchannel is idle even when the NAV timer is non-zero when the networkidentifier associated with setting the NAV timer corresponds to the AP114; whereas performing the virtual CCA includes concluding that thevirtual CCA indicates the communication channel is busy when the NAVtimer is non-zero and when the network identifier associated withsetting the NAV timer does not correspond to the AP 114. In anembodiment, performing the CCA operation includes not performing thevirtual CCA when the network identifier associated with setting the NAVtimer corresponds to the AP 114; whereas performing the CCA operationincludes performing the virtual CCA when the network identifierassociated with setting the NAV timer does not correspond to the AP 114.

If the network interface device 162 determines, at block 516, that thecommunication channel is not busy (e.g., the communication channel isidle), the flow proceeds to block 512, at which the network interfacedevice 162 transmits an NDP in response to the trigger frame received atblock 504.

On the other hand, if the network interface device 162 determines, atblock 516, that the communication channel is busy, the flow proceeds toblock 520. At block 520, the network interface device 162 does nottransmit an NDP in response to the trigger frame received at block 504.

Referring again to FIGS. 2A, 3A, and 3B, the MU ranging measurementexchange 200/300/350 may be included in a set of multiple MU rangingmeasurement exchanges 200/300/350 performed so as to obtain multipleranging measurements, according to some embodiments. As discussed above,the ranging measurement feedback in DL FB frames 244 may correspond toUL NDPs 224 transmitted in a previous MU ranging measurement exchange200/300/350 in the set of multiple MU ranging measurement exchanges200/300/350. Similarly, the ranging measurement feedback in UL FB frames268 may correspond to a DL NDP(s) 236 transmitted in a previous MUranging measurement exchange 200/300/350 in the set of multiple MUranging measurement exchanges 200/300/350.

In an embodiment, the AP 114 compares i) values of time of arrival (TOA)and time of departure (TOD) (e.g., t₁ and t₄) in the UL FB frames 268received in a particular MU ranging measurement exchange 200/300/350with ii) values of TOA and TOD (e.g., t₂ and t₃) recorded by the AP 114from previous MU ranging measurement exchanges 200/300/350 to determinethe appropriate TOA and TOD (e.g., t₂ and t₃) measurements recorded bythe AP 114 that correspond to the TOA and TOD (e.g., t₁ and t₄)measurements in the UL FB frames 268. Similarly, the client station 154compares i) values of TOA and TOD (e.g., t₂ and t₃) in the DL FB frames244 received in a particular MU ranging measurement exchange 200/300/350with ii) values of TOA and TOD (e.g., t₁ and t₄) recorded by the clientstation 154 from previous MU ranging measurement exchanges 200/300/350to determine the appropriate TOA and TOD (e.g., t₁ and t₄) measurementsrecorded by the client station 154 that correspond to the TOA and TOD(e.g., t₂ and t₃) measurements in the DL FB frames 244.

In another embodiment, the AP 114 selects different token values fordifferent MU ranging measurement exchanges 200/300/350 in the set ofmultiple MU ranging measurement exchanges 200/300/350. In an embodiment,the AP 114 includes the selected token value in a field of the triggerframe 216 and/or in a field of the NDPA frame 228. When the AP 114records TOA and TOD values (e.g., t₂ and t₃) in connection with aparticular MU ranging measurement exchange 200/300/350, the AP 114associates the recorded TOA and TOD values with the token valuecorresponding to the particular MU ranging measurement exchange200/300/350. When the AP 114 generates the DL FB frames 244 that includeTOA and TOD values (e.g., t₂ and t₃), the AP 114 includes in DL FBframes 244 the token value to which the TOA and TOD values correspond.

Similarly, the client station 154 notes the token value for a particularMU ranging measurement exchange 200/300/350 in the trigger frame 216and/or in the NDPA frame 228; and when the client station 154 recordsTOA and TOD values (e.g., t₁ and t₄) in connection with the particularMU ranging measurement exchange 200/300/350, the client station 154associates the recorded TOA and TOD values with the token valuecorresponding to the particular MU ranging measurement exchange200/300/350. When the client station 154 generates the UL FB frame 268that includes TOA and TOD values (e.g., t₁ and t₄), the client station154 includes in the UL FB frame 268 the token value to which the TOA andTOD values correspond.

When the client station 154 receives the DL FB frame 244, the clientstation 154 uses the token value in the DL FB frame 244 to determine theappropriate TOA and TOD (e.g., t₁ and t₄) measurements recorded by theclient station 154 that correspond to the TOA and TOD (e.g., t₂ and t₃)measurements in the DL FB frames 244. Similarly, when the AP 114receives the UL FB frame 268, the AP 114 uses the token value in the ULFB frame 268 to determine the appropriate TOA and TOD (e.g., t₂ and t₃)measurements recorded by the AP 114 that correspond to the TOA and TOD(e.g., t₁ and t₄) measurements in the DL FB frames 244.

FIG. 6 is a diagram of a set 600 of example single-user (SU) rangingmeasurement exchanges 604 in an SU ranging measurement procedure,according to an embodiment. The diagram of FIG. 6 is described in thecontext of the example network 110 merely for explanatory purposes. Insome embodiments, signals illustrated in FIG. 6 are generated by othersuitable communication devices in other suitable types of wirelessnetworks.

In an embodiment, the set 600 of SU ranging measurement exchanges 604 istransmitted in a single TXOP. In another embodiment, each SU rangingmeasurement exchange 604 is transmitted within a respective TXOP. Inanother embodiment, at least two SU ranging measurement exchanges 604are transmitted within a first TXOP, and one or more other SU rangingmeasurement exchanges 604 are transmitted within a second TXOP.

Each SU ranging measurement exchange 604 corresponds to aclient-initiated SU ranging measurement exchange, according to anembodiment. The SU ranging measurement exchange 604 includes an UL NDPportion 608 and a DL NDP portion 612. In the UL NDP transmission portion608, a first communication device (e.g., the client station 154)transmits a PPDU 616 that includes an SU UL NDPA having informationindicating the initiation of the SU ranging measurement exchange 604. Inan embodiment, the SU UL NDPA in the PPDU 616 is a type of NDPA framespecifically for initiating an SU ranging measurement exchange such asthe SU ranging measurement exchange 604. The SU UL NDPA in the PPDU 616causes the AP 114 to be ready to receive an NDP as part of an SU rangingmeasurement exchange.

The client station 154 then begins transmitting an UL NDP 620 a definedtime period after an end of the PPDU 616. In an embodiment, the definedtime period is SIFS. In other embodiments, another suitable time periodis utilized.

The UL NDP 620 includes PHY preambles having one or more STFs (e.g., anL-STF), one or more LTFs (e.g., an L-LTF and an NL-LTF) and one or moresignal fields (e.g., an L-SIG and an NL-SIG), in an embodiment. The ULNDP 620 omits a PHY data portion.

When transmitting the UL NDP 620, the client station 154 records a timet₁₁ at which the client station 154 began transmitting a particularportion the UL NDP 620 (e.g., the NL-LTF). Similarly, when the AP 114receives the UL NDP 620, the AP 114 records a time t₂ at which the AP114 began receiving the particular portion of the UL NDP 620 (e.g., theNL-LTF).

In some embodiments, when transmitting the UL NDP 620, the clientstation 154 (e.g., a client station 154 with multiple antennas 174)records an angle of departure, AoD₁, at which the UL NDP 616 left theantennas 178 of the client station 154. Similarly, when the AP 114receives the UL NDP 620, the AP 114 records an angle of arrival, AoA₁,at which the UL NDP 616 arrived at the antennas 138 of the AP 114.

The AP 114 generates a DL NDP 624 and, responsive to the UL NDP 620, theAP 114 begins transmitting the DL NDP 624 a defined time period after anend of reception of the UL NDP 620. In an embodiment, the defined timeperiod is SIFS. In other embodiments, another suitable time period isutilized. The DL NDP 624 includes a PHY preamble having one or moreSTFs, one or more LTFs (e.g., an L-LTF and an NL-LTF) and one or moresignal fields (e.g., an L-SIG and an NL-SIG), in an embodiment. The DLNDP 624 omits a PHY data portion.

When transmitting the DL NDP 624, the AP 114 records a time t₃ at whichthe AP 114 began transmitting a particular portion (e.g., the NL-LTF) ofthe DL NDP 624. Similarly, when the client station 154 receives the DLNDP 624, the client station 154 records a time t₄ at which the clientstation 154 began receiving a particular portion (e.g., the NL-LTF) ofthe DL NDP 624.

In some embodiments, when transmitting the DL NDP 624, the AP 114records an AoD₂ at which the DL NDP 624 left the antennas 138 of the AP114. Similarly, when the client station 154 receives the DL NDP 624, theclient station 154 records an AoA₂ at which the DL NDP 624 arrived atthe antennas 178 of the client station 154.

In an embodiment, the AP 114 transmits a DL PPDU 628 a defined timeperiod after an end of the DL NDP 624. In an embodiment, the definedtime period is SIFS. In other embodiments, another suitable time periodis utilized. The PPDU 628 corresponds to a ranging measurement feedbackpacket. The PPDU 628 includes the recorded times t₂ and t₃. In someembodiments, the PPDU 628 respectively includes the recorded angles AoA₁and AoD₂. In some embodiments, the PPDU 628 optionally also includesrespective channel estimate information determined by the AP 114 basedon reception of the UL NDP 620.

In another embodiment, the ranging measurement feedback packet in thePPDU 628 includes feedback information regarding a previous SU rangingmeasurement exchange 604. For example, the PPDU 628 includes therecorded times t₂ and t₃ of NDPs in the previous SU ranging measurementexchange 604. In some embodiments, the PPDU 628 respectively includesthe recorded angles AoA₁ and AoD₂ of NDPs in the previous SU rangingmeasurement exchange 604. In some embodiments, the PPDU 628 optionallyalso includes respective channel estimate information determined by theAP 114 based on reception of an UL NDP in the previous SU rangingmeasurement exchange 604.

After receipt of the PPDU 628, the client station 154 calculates atime-of-flight between the AP 114 and the client station 154 using therecorded times t₁, t₂, t₃, and t₄, according to an embodiment. Anysuitable technique, including currently known techniques, may beutilized to calculate a time-of-flight using the recorded times t₁, t₂,t₃, and t₄. A distance between the AP 114 and the client station 154 maybe calculated using the calculated times-of-flight, e.g., byrespectively multiplying the times-of-flight by the speed of light,according to an embodiment.

In some embodiments, the client station 154 calculates an estimatedposition of the client station using the calculated time-of-flight. Forexample, the client station 154 uses triangulation techniques tocalculate an estimated position of the client station 154 using thecalculated time-of-flight. In some embodiments, the client station 154calculates an estimated positions of the client station also using therecorded angles AoD₁, AoA₁, AoD₂, and AoA₂. For example, the recordedangles AoD₁, AoA₁, AoD₂, and AoA₂ are used as part of a triangulationalgorithm for determining positions of communication devices.

In an embodiment, the client station 154 compares i) values of TOA andTOD (e.g., t₁ and t₄) in the DL FB frame 628 received in a particular SUranging measurement exchange 604 with ii) values of TOA and TOD (e.g.,t₂ and t₃) recorded by the client station 154 from previous SU rangingmeasurement exchanges 604 to determine the appropriate TOA and TOD(e.g., t₁ and t₄) measurements recorded by the client station 154 thatcorrespond to the TOA and TOD (e.g., t₂ and t₃) measurements in the ULFB frame 628.

In another embodiment, the client station 154 selects different tokenvalues for different SU ranging measurement exchanges 604 in the set 600of multiple SU ranging measurement exchanges 604. In an embodiment, theclient station 154 includes the selected token value in a field of theUL NDPA frame 616. When the client station 154 records TOA and TODvalues (e.g., t₁ and t₄) in connection with a particular SU rangingmeasurement exchange 604, the client station 154 associates the recordedTOA and TOD values with the token value corresponding to the particularSU ranging measurement exchange 604.

Similarly, the AP 114 notes the token value for a particular SU rangingmeasurement exchange 604 in the NDPA frame 616; and when the AP 114records TOA and TOD values (e.g., t₂ and t₃) in connection with theparticular SU ranging measurement exchange 604, the AP 114 associatesthe recorded TOA and TOD values with the token value corresponding tothe particular SU ranging measurement exchange 604. When the AP 114generates the DL FB frame 628 that includes TOA and TOD values (e.g., t₂and t₃), the AP 114 includes in the DL FB frame 628 the token value towhich the TOA and TOD values correspond.

When the client station 154 receives the DL FB frame 628, the clientstation 154 uses the token value in the DL FB frame 628 to determine theappropriate TOA and TOD (e.g., t₁ and t₄) measurements recorded by theclient station 154 that correspond to the TOA and TOD (e.g., t₂ and t₃)measurements in the DL FB frames 628.

Responsive to receipt of the DL PPDU 628, the client station 154generates an UL PPDU (not shown) that includes an ACK frame, accordingto an embodiment. The client station 154 transmits the UL PPDU havingthe ACK frame a defined time period after an end of the DL PPDU 628. Inan embodiment, the defined time period is SIFS. In other embodiments,another suitable time period is utilized.

In another embodiment, the client station 154 does not generate andtransmit an UL PPDU having an ACK frame even when the client station 154successfully receives the DL PPDU 628.

In an embodiment, the network interface device 122 of the AP 114transmits the DL NDP 624 a defined time period (e.g., SIFS) after an endof reception of the UL NDP 620 and in response to the UL NDP 620. In anembodiment, the network interface device 122 transmits the DL NDP 624without first performing a CCA operation in connection with transmittingthe DL NDP 624. In an embodiment, the network interface device 122transmits the DL NDP 624 regardless of whether a communication channel(via which the NDP is being transmitted) is busy.

In another embodiment, the network interface device 162 performs a CCAoperation in connection with transmitting the DL NDP 624 and does nottransmit the DL NDP 624 if the CCA operation indicates that thecommunication channel is busy; whereas the network interface device 162transmits the DL NDP 624 if the CCA operation indicates that thecommunication channel is idle. In an embodiment, the CCA operationincludes a physical CCA and a virtual CCA. In an embodiment, the networkinterface device 162 performs the physical CCA during a time periodimmediately prior to reception of the UL NDPA 616. In an embodiment, thetime period immediately prior to reception of the UL NDPA 616 has aduration equal to a point coordination function (PCF) interframe space(PIFS) as defined by the IEEE 802.11 Standard or another suitable timeduration, such as SIFS. In an embodiment, the network interface device162 performs the physical CCA during a time period prior to reception ofthe UL NDP 620. In an embodiment, the time period prior to reception ofthe UL NDP 620 has a duration equal to SIFS or another suitable timeduration. In an embodiment, the network interface device 162 performsthe physical CCA during a time period prior to transmission of the DLNDP 624. In an embodiment, the time period prior to transmission of theDL NDP 624 has a duration equal to SIFS or another suitable timeduration.

In an embodiment, performing the virtual CCA includes concluding thatthe virtual CCA indicates the communication channel is idle even whenthe NAV timer is non-zero when the network identifier associated withsetting the NAV timer corresponds to the client station 154; whereasperforming the virtual CCA includes concluding that the virtual CCAindicates the communication channel is busy when the NAV timer isnon-zero and when the network identifier associated with setting the NAVtimer does not correspond to the client station 154. In an embodiment,performing the CCA operation includes not performing the virtual CCAwhen the network identifier associated with setting the NAV timercorresponds to the client station 154; whereas performing the CCAoperation includes performing the virtual CCA when the networkidentifier associated with setting the NAV timer does not correspond tothe client station 154.

In an embodiment, each SU ranging measurement exchange 604 correspondsto a frame exchange, i.e., a sequence of frames. In an embodiment, ifthe client station 154 receives both the DL NDP 624 and DL FB 628correctly, the client station 154 determines i) that the SU rangingmeasurement was successful, and ii) that the frame exchange wassuccessful. In an embodiment, if the client station 154 receives the DLNDP 624 correctly but does not receive the DL FB 628 correctly, theclient station 154 determines i) that the SU ranging measurement was notsuccessful, but ii) that the frame exchange was successful. In anembodiment, if the client station 154 does not receive the DL NDP 624correctly but receives the DL FB 628 correctly, the client station 154determines i) that the SU ranging measurement was not successful, butii) that the frame exchange was successful. In an embodiment, if theclient station 154 does not receive the DL NDP 624 correctly and alsodoes not receive the DL FB 628 correctly, the client station 154determines i) that the SU ranging measurement was not successful, andii) that the frame exchange was not successful. In another embodiment,if the client station 154 does not receive the DL NDP 624 correctlyand/or does not receive the DL FB 628 correctly, the client station 154determines i) that the SU ranging measurement was not successful, andii) that the frame exchange was not successful.

In an embodiment, when the client station 154 determines that the SUranging measurement was not successful, the client station 154 does notuse any measurements made in connection with the SU ranging measurementexchange 604 for determining a time of flight and/or a distance. In anembodiment, when the client station 154 determines that the frameexchange was not successful was not successful, the client station 154does not use any measurements made in connection with the SU rangingmeasurement exchange 604 for determining a time of flight and/or adistance.

In some embodiments, the client station 154 and/or the AP 114 employs abackoff procedure in connection with one or more transmissions in theset 600 of SU ranging measurement exchanges 604, at least in somesituations. In an embodiment, the backoff process involves the use of abackoff counter or timer. While the communication device determines thatthe channel medium is idle, the communication device decrements thebackoff timer. When the communication device determines that thecommunication medium is busy, the communication device pauses thebackoff timer and does not resume decrementing the backoff timer untilthe communication medium is subsequently determined to be idle. Thebackoff timer is set to a value chosen randomly or pseudo-randomly sothat backoff timers of different communication devices in the networktend to reach zero at different times. Generally, if the communicationmedium is still idle when the backoff timer reaches zero, thecommunication device determines that the communication device is free totransmit. On the other hand, if the communication medium is busy whenthe backoff timer reaches zero, the communication device resets thebackoff timer and the process repeats. In an embodiment, determiningwhether the channel medium is idle includes measuring an energy level inthe channel medium and comparing the measured energy level to athreshold. When the measured energy level is less than the threshold,the channel medium is determined to be idle; whereas when the measuredenergy level meets the threshold, the channel medium is determined to bebusy, according to an embodiment.

In an embodiment, setting the backoff timer includes randomly orpseudorandomly choosing an initial value for the backoff timer from arange of initial values. In an embodiment, the range of initial valuesis [0, CW], where CW is a contention window parameter, where the initialvalue and CW are in units of a slots, and where each slot corresponds toa suitable time period. For example, the IEEE 802.11 Standard definesslot times of 20 microseconds (IEEE 802.11b) and 9 microseconds (IEEE802.11a, 11n, and 11ac), where different slot times are used fordifferent versions of the protocol. In an embodiment, CW is initiallyset to a minimum value CWmin. However, after each failed transmissionattempt (e.g., failure to receive an acknowledgment of thetransmission), the value of CW is approximately doubled with an upperbound of CWmax. The parameters CWmin and CWmax are also in units ofslots. In an embodiment, the backoff timer is decremented in units ofslots.

In some embodiments, the client station 154 and/or the AP 114 employs anerror recovery procedure (sometimes referred to herein as the “PIFSrecovery procedure”) in connection with one or more transmissions in theset 600 of SU ranging measurement exchanges 604, at least in somesituations. In an embodiment, the PIFS recovery procedure involvesdetermining whether the channel medium is idle for a time period thatcorresponds to a slot time after the channel medium transitioned frombeing busy to being idle. When the channel medium is determined to beidle for the slot time after the channel medium transitioned from beingbusy to being idle, the PIFS recovery procedure indicates that a next SUranging measurement exchanges 604 in the set 600 can commence, accordingto an embodiment.

In an embodiment, when the client station 154 determines that a frameexchange 604 was successful, the client station 154 determines that anext occurring frame exchange 604 can commence without first performinga backoff procedure if there is adequate time left in a TXOP for thenext occurring frame exchange 604. For example, when the client station154 determines that the frame exchange 604 was successful and when thereis adequate time left in a TXOP for the next occurring frame exchange604, the client station 154 determines that the UL NDPA 616 in the nextoccurring frame exchange 604 can be transmitted a predetermined timeperiod (e.g., SIFS or another suitable time period) after reception ofthe DL FB 628 in the frame exchange 604 has ended.

In an embodiment, when the client station 154 determines that a firstoccurring frame exchange 604 was successful, the client station 154determines that a next occurring frame exchange 604 can commence withoutfirst performing a backoff procedure if there is adequate time left in aTXOP for the next occurring frame exchange 604. For example, when theclient station 154 determines that the first occurring frame exchange604 was successful and when there is adequate time left in a TXOP forthe next occurring frame exchange 604, the client station 154 determinesthat the UL NDPA 616 in the next occurring frame exchange 604 can betransmitted a predetermined time period (e.g., SIFS or another suitabletime period) after reception of the DL FB 628 in the first occurringframe exchange 604 has ended.

In an embodiment, when the client station 154 determines i) that a frameexchange 604 was successful and ii) that there is adequate time left ina TXOP for a next occurring frame exchange 604, the client station 154determines that the next occurring frame exchange 604 can commence afterperforming a backoff procedure using CWmin (e.g., the backoff timer isinitialized with CWmin, and the backoff timer decrements for each slottime during which the communication medium is idle). For example, whenthe client station 154 determines that a frame exchange 604 wassuccessful and if there is adequate time left in a TXOP for a nextoccurring frame exchange 604, the client station 154 determines that theUL NDPA 616 in the next occurring frame exchange 604 can be transmittedafter performing a backoff procedure using CWmin.

FIG. 7 is a flow diagram of an example method 700 performed by acommunication device (e.g., performed by the network interface 162 ofthe client station 154) in connection with an SU ranging measurementexchange 604, according to an embodiment. FIG. 7 is described in thecontext of the network interface device 162 of the client station 154merely for explanatory purposes. In other embodiments, the method 700 isperformed by another suitable communication device (e.g., the networkinterface 122 of the AP 114 or another suitable network interfacedevice). FIG. 7 is also described in the context of the set 600 of SUranging measurement exchanges 604 of FIG. 6 merely for explanatorypurposes. In other embodiments, the method 700 is performed in thecontext of other suitable SU ranging measurement exchanges.

The method 700 commences in connection with transmission of the UL NDPA616 and the UL NDP 620. At block 704, the network interface device 162determines whether the network interface device 162 has begun receivinga PPDU during a predetermined time period after an end of transmissionof the UL NDP 620. In an embodiment, the predetermined time periodcorresponds to an expected time in which to begin receiving the DL NDP624 as part of the frame exchange 604. In an embodiment, thepredetermined time period corresponds toaSIFSTime+aSlotTime+aRxPHYStartDelay, where: aSIFSTime is a nominal time(e.g., in microseconds) required by wireless network interfaces in thenetwork 110 (FIG. 1) in order to receive a last symbol of a frame on acommunication medium, process the frame, and respond with a first symbolon the communication medium of an earliest possible response frame;aSlotTime is a Slot Time (e.g., in microseconds) used by networkinterfaces in the network 110; and aRxPHYStartDelay is a delay (e.g., inmicroseconds) from the start of the PPDU at a receiver's antenna to anissuance of an indication primitive by the PHY processor 170 (FIG. 1)that the PHY processor 170 began receiving a PPDU.

If the network interface device 162 determines that the networkinterface device 162 did not begin receiving a PPDU during thepredetermined time period after the end of transmission of the UL NDP620, the flow proceeds to block 708. At block 708, the client station154 determines that the frame exchange 604 failed and performs an errorrecovery procedure in accordance with a frame exchange failure. Forexample, in an embodiment, the network interface device 162 performs abackoff procedure and retransmits the UL NDPA 616 and the UL NDP 620 inconnection with performing the backoff procedure. In an embodiment, thenetwork interface device 162 performs the backoff procedure andretransmits the UL NDPA 616 and the UL NDP 620 in a subsequent TXOP,e.g., in a TXOP that is different than a TXOP in which the UL NDPA 616and the UL NDP 620 were previously transmitted. In another embodiment,the network interface device 162 performs a backoff procedure andtransmits, in connection with performing the backoff procedure, a ULNDPA 616 and a UL NDP 620 corresponding to a next frame exchange 604. Inan embodiment, the network interface device 162 performs the backoffprocedure and transmits the UL NDPA 616 and the UL NDP 620 in asubsequent TXOP, e.g., in a TXOP that is different than a TXOP in whichthe UL NDPA 616 and the UL NDP 620 for the previous frame exchange 604were previously transmitted. In an embodiment, when transmitting the ULNDPA 616 in a subsequent TXOP, the backoff procedure includes settingthe backoff timer to CWmin.

On the other hand, if the network interface device 162 determines, atblock 704, that the network interface device 162 began receiving a PPDUduring the predetermined time period after the end of transmission ofthe UL NDP 620, the flow proceeds to block 712. At block 712, thenetwork interface device 162 determines whether the network interfacedevice 162 received the DL NDP 624 correctly. If the network interfacedevice 162 determines that the network interface device 162 did notreceive the DL NDP 624 correctly, the flow proceeds to block 708.

On the other hand, if the network interface device 162 determines, atblock 712, that the network interface device 162 received the DL NDP 624correctly, the flow proceeds to block 716. At block 716, the networkinterface device 162 determines whether the network interface device 162received the DL FB 628 correctly. If the network interface device 162determines that the network interface device 162 received the DL FB 628correctly, the flow proceeds to block 720.

At block 720, the network interface device 720 commences a next SUranging measurement exchange 604 in the set 600. In an embodiment, thenetwork interface device 720 commences the next SU ranging measurementexchange 604 if there is enough time remaining in a TXOP to complete thenext SU ranging measurement exchange 604. In an embodiment, the networkinterface device 720 commences the next frame exchange 604 without firstperforming a backoff procedure if there is adequate time left in theTXOP for the next occurring frame exchange 604. For example, when thenetwork interface device 720 determines there is adequate time left in aTXOP for the next occurring frame exchange 604, the network interfacedevice 720 determines that the UL NDPA 616 in the next occurring frameexchange 604 can be transmitted a predetermined time period (e.g., SIFSor another suitable time period) after reception of the DL FB 628 in theframe exchange 604 has ended.

On the other hand, if the network interface device 162 determines, atblock 716, that the network interface device 162 did not receive the DLFB 628 correctly, the flow proceeds to block 724. At block 724, theclient station 154 determines that the ranging measurement failed andperforms an error recovery procedure in accordance with a rangingmeasurement failure. For example, in an embodiment, the networkinterface device 162 performs a backoff procedure and transmits, inconnection with performing the backoff procedure, a UL NDPA 616 and a ULNDP 620 corresponding to a next frame exchange 604 if there is adequatetime in the TXOP to complete the next frame exchange 604. In anembodiment, if there is not adequate time in the TXOP to complete thenext frame exchange 604, the network interface device 162 performs thebackoff procedure and transmits the UL NDPA 616 and the UL NDP 620 ofthe next frame exchange 604 in a subsequent TXOP. In an embodiment, whentransmitting the UL NDPA 616 in a subsequent TXOP, the backoff procedureincludes setting the backoff timer to CWmin.

In another embodiment, block 724 includes the network interface device162 performing a PIFS recovery procedure and transmitting, in connectionwith performing the PIFS recovery procedure, a UL NDPA 616 and a UL NDP620 corresponding to a next frame exchange 604 if there is adequate timein the TXOP to complete the next frame exchange 604. In an embodiment,if there is not adequate time in the TXOP to complete the next frameexchange 604, the network interface device 162 performs the backoffprocedure and transmits the UL NDPA 616 and the UL NDP 620 of the nextframe exchange 604 in a subsequent TXOP. In an embodiment, whentransmitting the UL NDPA 616 in a subsequent TXOP, the backoff procedureincludes setting the backoff timer to CWmin.

In another embodiment, if the network interface device 162 determines,at block 716, that the network interface device 162 did not receive theDL FB 628 correctly, the flow proceeds to block 708, and block 724 isomitted.

FIG. 8 is a flow diagram of another example method 800 performed by acommunication device (e.g., performed by the network interface 162 ofthe client station 154) in connection with an SU ranging measurementexchange 604, according to an embodiment. FIG. 8 is described in thecontext of the network interface device 162 of the client station 154merely for explanatory purposes. In other embodiments, the method 800 isperformed by another suitable communication device (e.g., the networkinterface 122 of the AP 114 or another suitable network interfacedevice). FIG. 8 is also described in the context of the set 600 of SUranging measurement exchanges 604 of FIG. 6 merely for explanatorypurposes. In other embodiments, the method 800 is performed in thecontext of other suitable SU ranging measurement exchanges.

The method 800 is similar to the method 700 of FIG. 7, and like-numberedelements are not discussed in detail for purposes of brevity.

Similar to the method 700 of FIG. 7, the method 800 commences inconnection with transmission of the UL NDPA 616 and the UL NDP 620.

At block 712, if the network interface device 162 determines that thenetwork interface device 162 did not receive the DL NDP 624 correctly,the flow proceeds to block 804. At block 804, the network interfacedevice 162 determines whether the network interface device 162 receivedthe DL FB 628 correctly. If the network interface device 162 determines,at block 804, that the network interface device 162 received the DL FB628 correctly, the flow proceeds to block 720. On the other hand, if thenetwork interface device 162 determines, at block 804, that the networkinterface device 162 did not receive the DL FB 628 correctly, the flowproceeds to block 724.

FIG. 9 is a flow diagram of an example method 900 performed by acommunication device (e.g., performed by the network interface 122 ofthe AP 114) in connection with an SU ranging measurement exchange 604,according to an embodiment. FIG. 9 is described in the context of thenetwork interface device 122 of the AP 114 merely for explanatorypurposes. In other embodiments, the method 900 is performed by anothersuitable communication device (e.g., the network interface 162 of theclient station 154 or another suitable network interface device). FIG. 9is also described in the context of the set 600 of SU rangingmeasurement exchanges 604 of FIG. 6 merely for explanatory purposes. Inother embodiments, the method 900 is performed in the context of othersuitable SU ranging measurement exchanges.

The method 900 commences in connection with reception of the UL NDPA 616by the AP 114. At block 904, the network interface device 122 determineswhether the network interface device 122 received the UL NDP 620correctly. In an embodiment, block 904 includes determining whether thenetwork interface device 122 began receiving a PPDU during apredetermined time period after an end of reception of the UL NDPA 616.In an embodiment, the predetermined time period corresponds to anexpected time in which to begin receiving the UL NDP 620 as part of theframe exchange 604. In an embodiment, the predetermined time periodcorresponds to aSIFSTime+aSlotTime. If the network interface device 122determines that the network interface device 122 did not begin receivinga PPDU during the predetermined time period after an end of transmissionof the UL NDPA 616, the network interface device 122 determines that thenetwork interface device 122 did not receive the UL NDP 620 correctly.

On the other hand, in an embodiment, the network interface device 122determines that the network interface device 122 began receiving a PPDUduring the predetermined time period after an end of transmission of theUL NDPA 616, the network interface device 122 determines whether the ULNDP 620 was received correctly.

If the network interface device 122 determines that the UL NDP 620 wasreceived correctly, the flow proceeds to block 908. At block 908, thenetwork interface device 122 transmits the DL NDP 624 as part of the SUranging measurement exchange. At block 912, the network interface device122 transmits the DL FB 628 as part of the SU ranging measurementexchange.

On the other hand, if the network interface device 122 determines thatthe UL NDP 620 was not received correctly, the flow proceeds to block916. At block 916, the network interface device 122 does not transmitthe DL NDP 624. Additionally, the network interface device 122 does nottransmit the DL FB 628.

FIG. 10 is a flow diagram of an example method for performing rangemeasurement exchanges (e.g., SU range measurement exchanges), accordingto an embodiment. FIG. 10 is described in the context of the networkinterface device 162 of the client station 154 merely for explanatorypurposes. In other embodiments, the method 1000 is performed by anothersuitable communication device (e.g., the network interface 122 of the AP114 or another suitable network interface device). FIG. 10 is alsodescribed in the context of the set 600 of SU ranging measurementexchanges 604 of FIG. 6 merely for explanatory purposes. In otherembodiments, the method 1000 is performed in the context of othersuitable ranging measurement exchanges.

At block 1004, the network interface device 162 transmits an NDPA (e.g.,the UL NDPA 616) and an NDP (e.g., the UL NDP 620) as part of a rangingmeasurement exchange (e.g., the SU ranging measurement exchange 604)among a set (e.g., the set 600) of ranging measurement exchanges. In anembodiment, the network interface device 162 transmits the NDPA and theNDP during a TXOP.

In an embodiment, block 1004 includes generating the NDPA to include atoken that is associated with the ranging measurement exchange and thatis different than tokens associated with other ranging measurementexchanges in the set of ranging measurement exchanges.

Block 1004 includes recording a ToD (e.g., t₁) corresponding to thetransmission of the NDP. In an embodiment in which the network interfacedevice 162 associates the ranging measurement exchange with a token,block 1004 includes the network interface device 162 associating therecorded ToD with the token that is associated with the rangingmeasurement exchange.

At block 1008, the network interface device 162 determines whether anNDP (e.g., the DL NDP 624) was received i) in response to the NDPtransmitted at block 1004 and ii) as part of the ranging measurementexchange. In an embodiment, block 1008 includes determining whether thenetwork interface device 162 began receiving a PPDU during apredetermined time period after an end of transmission of the NDP atblock 1004. In an embodiment, the predetermined time period correspondsto an expected time in which to begin receiving the DL NDP 624 as partof the frame exchange 604. In an embodiment, the predetermined timeperiod corresponds to aSIFSTime+aSlotTime+aRxPHYStartDelay. In anembodiment, if the network interface device 162 determines that thenetwork interface device 162 did not begin receiving a PPDU during thepredetermined time period after the end of transmission of the NDP atblock 1004, the network interface device 162 determines, at block 1008,that the network interface device 162 did not receive the DL NDPcorrectly.

In an embodiment, when an NDP is received in connection with block 1008,block 1008 includes determining whether the network interface device 162can correctly decode one or more signal fields in a PHY preamble of thereceived NDP. In an embodiment, if the network interface device 162cannot correctly decode the one or more signal fields in the PHYpreamble of the received NDP, the network interface device 162determines, at block 1008, that the network interface device 162 did notreceive the DL NDP correctly. On the other hand, if the networkinterface device 162 can correctly decode the one or more signal fieldsin the PHY preamble of the received NDP, the network interface device162 determines, at block 1008, that the network interface device 162received the DL NDP correctly, according to an embodiment.

If the network interface device 162 determines, at block 1008, that thenetwork interface device 162 received the DL NDP correctly, the flowproceeds to block 1012. At block 1012, the network interface device 162records a ToA (e.g., t₄) corresponding to the reception of the NDP atblock 1008. In an embodiment in which the network interface device 162associates the ranging measurement exchange with a token, block 1008includes the network interface device 162 associating the recorded ToAwith the token that is associated with the ranging measurement exchange.

At block 1016, the network interface device 162 determines whether aPPDU including ranging measurement feedback (e.g., the DL FB 628) wasreceived as part of the ranging measurement exchange. In an embodiment,block 1016 includes determining whether the network interface device 162began receiving a PPDU during a predetermined time period after an endof reception of the NDP corresponding to block 1008. In an embodiment,the predetermined time period corresponds to an expected time in whichto begin receiving the DL FB 628 as part of the frame exchange 604. Inan embodiment, the predetermined time period corresponds toaSIFSTime+aSlotTime. In an embodiment, if the network interface device162 determines that the network interface device 162 did not beginreceiving a PPDU during the predetermined time period after the end ofreception of the NDP corresponding to block 1008, the network interfacedevice 162 determines, at block 1016, that the network interface device162 did not receive the PPDU having the ranging measurement feedbackcorrectly.

In an embodiment, when a PPDU is received in connection with block 1016,block 1016 includes determining whether the network interface device 162can correctly decode information in the PPDU correctly. In anembodiment, if the network interface device 162 cannot correctly decodeinformation in the PPDU, the network interface device 162 determines, atblock 1016, that the network interface device 162 did not receive thePPDU including ranging measurement feedback (e.g., the DL FB 628)correctly. On the other hand, if the network interface device 162 cancorrectly decode the information in PPDU, the network interface device162 determines, at block 1016, that the network interface device 162received the PPDU including ranging measurement feedback (e.g., the DLFB 628) correctly, according to an embodiment.

If the network interface device 162 determines, at block 1016, that thenetwork interface device 162 received the PPDU including rangingmeasurement feedback (e.g., the DL FB 628) correctly, the flow proceedsto block 1020. At block 1020, the network interface device 162 uses theranging measurement feedback received in connection with block 1016 tocalculate a distance measurement. In an embodiment in which the PPDUthat includes the ranging measurement feedback received in connectionwith block 1016 includes a token, block 1020 includes using the token inthe PPDU to determine a recorded ToD and a recorded ToA that correspondsto the token.

At block 1024, the network interface device 162 commences a next rangingmeasurement exchange in the set of ranging measurement exchanges. In anembodiment, if there is adequate time in the TXOP to complete the nextranging measurement exchange in the set (e.g., the set 600), the networkinterface device 162 commences the next ranging measurement exchangewithout performing a backoff procedure. In an embodiment, if there isnot adequate time in the TXOP to complete the next ranging measurementexchange in the set (e.g., the set 600), the network interface device162 commences the next ranging measurement exchange in a subsequent TXOPafter performing a backoff procedure. In an embodiment, if there is notadequate time in the TXOP to complete the next ranging measurementexchange in the set (e.g., the set 600), the network interface device162 commences the next ranging measurement exchange in a subsequent TXOPafter performing a backoff procedure in which CW is set to CWmin.

If the network interface device 162 determines, at block 1016, that thenetwork interface device 162 did not receive the PPDU including rangingmeasurement feedback (e.g., the DL FB 628) correctly, the flow proceedsto block 1028. At block 1028, the network interface device 162 commencesa next ranging measurement exchange in the set of ranging measurementexchanges. In an embodiment, if there is adequate time in the TXOP tocomplete the next ranging measurement exchange in the set (e.g., the set600), the network interface device 162 commences the next rangingmeasurement exchange after performing a backoff procedure. In anembodiment, if there is adequate time in the TXOP to complete the nextranging measurement exchange in the set (e.g., the set 600), the networkinterface device 162 commences the next ranging measurement exchangeafter performing a PIFS recovery procedure. In an embodiment, if thereis not adequate time in the TXOP to complete the next rangingmeasurement exchange in the set (e.g., the set 600), the networkinterface device 162 commences the next ranging measurement exchange ina subsequent TXOP after performing a backoff procedure.

If the network interface device 162 determines, at block 1008, that thenetwork interface device 162 did not receive the NDP (e.g., the DL NDP624) correctly, the flow proceeds to block 1032. At block 1032, thenetwork interface device 162 commences a next ranging measurementexchange in the set of ranging measurement exchanges. In an embodiment,if there is adequate time in the TXOP to complete the next rangingmeasurement exchange in the set (e.g., the set 600), the networkinterface device 162 commences the next ranging measurement exchangeafter performing a backoff procedure. In an embodiment, if there isadequate time in the TXOP to complete the next ranging measurementexchange in the set (e.g., the set 600), the network interface device162 commences the next ranging measurement exchange after performing aPIFS recovery procedure. In an embodiment, if there is not adequate timein the TXOP to complete the next ranging measurement exchange in the set(e.g., the set 600), the network interface device 162 commences the nextranging measurement exchange in a subsequent TXOP after performing abackoff procedure.

Embodiment 1

A method, comprising: transmitting, by a first communication device, afirst physical layer protocol data units (PPDU) that includes a firstnull data packet announcement (NDPA) frame as part of a first rangingmeasurement exchange, wherein the first ranging measurement exchange isamong a set of ranging measurement exchanges; transmitting, by the firstcommunication device, a first null data packet (NDP) as part of thefirst ranging measurement exchange; recording, by the firstcommunication device, a transmit time of the first NDP; determining, bythe first communication device, whether a second NDP was receivedcorrectly from a second communication device as part of the firstranging measurement exchange; and in response to determining that thesecond NDP was not received correctly, commencing, by the firstcommunication device, a second ranging measurement exchange, includingtransmitting a second PPDU that includes a second NDPA frame as part ofthe second ranging measurement exchange, wherein the second rangingmeasurement exchange is among the set of ranging measurement exchanges.

Embodiment 2

The method of embodiment 1, wherein: determining whether the second NDPwas received correctly includes determining, by the first communicationdevice, whether the first communication device began receiving thesecond NDP within an expected time period; the method further comprises:in response to determining that the first communication device did notbegin receiving the second NDP within the expected time period,performing, by the first communication device, a backoff operation; andthe second ranging measurement exchange is commenced after the backoffoperation.

Embodiment 3

The method of embodiment 1, wherein: determining whether the second NDPwas received correctly includes determining, by the first communicationdevice, whether the first communication device began receiving thesecond NDP within an expected time period; the method further comprises:in response to determining that the first communication device did notbegin receiving the second NDP within the expected time period,determining, by the first communication device, whether a channel mediumis idle for a predetermined time period; and the second rangingmeasurement exchange is commenced in response to determining that thechannel medium is idle for the predetermined time period.

Embodiment 4

The method of any of embodiments 1-3 wherein: determining whether thesecond NDP was received correctly includes determining, by the firstcommunication device, whether the first communication device correctlydecoded one or more signal fields in the second NDP; the method furthercomprises: in response to determining that the first communicationdevice did not correctly decode the one or more signal fields in thesecond NDP, performing, by the first communication device, a backoffoperation; and the second ranging measurement exchange is commencedafter the backoff operation.

Embodiment 5

The method of any of embodiments 1-3, wherein: determining whether thesecond NDP was received correctly includes determining, by the firstcommunication device, whether the first communication device correctlydecoded one or more signal fields in the second NDP; the method furthercomprises: in response to determining that the first communicationdevice did not correctly decode the one or more signal fields in thesecond NDP, determining, by the first communication device, whether achannel medium is idle for a predetermined time period; and the secondranging measurement exchange is commenced in response to determiningthat the channel medium is idle for the predetermined time period.

Embodiment 6

The method of any of embodiments 1-5, further comprising: receiving, bythe first communication device, the second NDP correctly; recording, bythe first communication device, a receive time of the second NDP;determining, by the first communication device, whether a third PPDU wasreceived correctly as part of the first ranging measurement exchange,wherein the third PPDU includes ranging measurement feedback from thesecond communication device; and in response to determining that thethird PPDU was not received correctly, commencing, by the firstcommunication device, the second ranging measurement exchange, includingtransmitting the second PPDU that includes the second NDPA frame as partof the second ranging measurement exchange.

Embodiment 7

The method of embodiment 6, further comprising: in response todetermining that the third PPDU was not received correctly, performing,by the first communication device, a backoff operation; and the secondranging measurement exchange is commenced after the backoff operation.

Embodiment 8

The method of embodiment 6, further comprising: in response to that thethird PPDU was not received correctly, determining, by the firstcommunication device, whether a channel medium is idle for apredetermined time period; and the second ranging measurement exchangeis commenced in response to determining that the channel medium is idlefor the predetermined time period.

Embodiment 9

The method of any of embodiments 6-8, further comprising: in response todetermining that the third PPDU was received correctly, commencing, bythe first communication device, the second ranging measurement exchangea predetermined time period after an end of reception of the third PPDUwithout determining whether a channel medium is idle.

Embodiment 10

The method of any of embodiments 1-9, further comprising: calculating,by the first communication device, a distance between the firstcommunication device and the second communication device, usingmeasurements corresponding to the set of ranging measurement exchanges.

Embodiment 11

An apparatus, comprising: a network interface device associated with afirst communication device, wherein the network interface devicecomprises one or more integrated circuit (IC) devices. The one or moreIC devices are configured to: transmit a first physical layer protocoldata units (PPDU) that includes a first null data packet announcement(NDPA) frame as part of a first ranging measurement exchange, whereinthe first ranging measurement exchange is among a set of rangingmeasurement exchanges; transmit a first null data packet (NDP) as partof the first ranging measurement exchange; record a transmit time of thefirst NDP; determine whether a second NDP was received correctly from asecond communication device as part of the first ranging measurementexchange; and in response to determining that the second NDP was notreceived correctly, commence a second ranging measurement exchange,including transmitting a second PPDU that includes a second NDPA frameas part of the second ranging measurement exchange, wherein the secondranging measurement exchange is among the set of ranging measurementexchanges.

Embodiment 12

The apparatus of embodiment 11, wherein the one or more IC devices areconfigured to: as part of determining whether the second NDP wasreceived correctly, determine whether the first communication devicebegan receiving the second NDP within an expected time period; inresponse to determining that the first communication device did notbegin receiving the second NDP within the expected time period, performa backoff operation; and commence the second ranging measurementexchange after the backoff operation.

Embodiment 13

The apparatus of embodiment 11, wherein the one or more IC devices areconfigured to: as part of determining whether the second NDP wasreceived correctly, determine whether the first communication devicebegan receiving the second NDP within an expected time period; inresponse to determining that the first communication device did notbegin receiving the second NDP within the expected time period,determine whether a channel medium is idle for a predetermined timeperiod; and commence the second ranging measurement exchange in responseto determining that the channel medium is idle for the predeterminedtime period.

Embodiment 14

The apparatus of any of embodiments 11-13, wherein the one or more ICdevices are configured to: as part of determining whether the second NDPwas received correctly, determine whether the first communication devicecorrectly decoded one or more signal fields in the second NDP; inresponse to determining that the first communication device did notcorrectly decode the one or more signal fields in the second NDP,perform backoff operation; and commence the second ranging measurementexchange after the backoff operation.

Embodiment 15

The apparatus of any of embodiments 11-13, wherein the one or more ICdevices are configured to: as part of determining whether the second NDPwas received correctly, determine whether the first communication devicecorrectly decoded one or more signal fields in the second NDP; inresponse to determining that the first communication device did notcorrectly decode the one or more signal fields in the second NDP,determine whether a channel medium is idle for a predetermined timeperiod; and commence the second ranging measurement exchange in responseto determining that the channel medium is idle for the predeterminedtime period.

Embodiment 16

The apparatus of any of embodiments 11-15, wherein the one or more ICdevices are configured to: record a receive time of the second NDP;determine a third PPDU was received correctly as part of the firstranging measurement exchange, wherein the third PPDU includes rangingmeasurement feedback from the second communication device; and inresponse to determining that the third PPDU was not received correctly,commence the second ranging measurement exchange, including transmittingthe second PPDU that includes the second NDPA frame as part of thesecond ranging measurement exchange.

Embodiment 17

The apparatus of embodiment 16, wherein the one or more IC devices areconfigured to: in response to determining that the third PPDU was notreceived correctly, perform a backoff operation; and commence the secondranging measurement exchange after the backoff operation.

Embodiment 18

The apparatus of embodiment 16, wherein the one or more IC devices areconfigured to: in response to that the third PPDU was not receivedcorrectly, determine whether a channel medium is idle for apredetermined time period; and commence the second ranging measurementexchange in response to determining that the channel medium is idle forthe predetermined time period.

Embodiment 19

The apparatus of any of embodiments 16-18, wherein the one or more ICdevices are configured to: in response to determining that the thirdPPDU was received correctly, commence the second ranging measurementexchange a predetermined time period after an end of reception of thethird PPDU without determining whether a channel medium is idle.

Embodiment 20

The apparatus of any of embodiments 11-19, wherein the one or more ICdevices are configured to: calculate a distance between the firstcommunication device and the second communication device, usingmeasurements corresponding to the set of ranging measurement exchanges.

At least some of the various blocks, operations, and techniquesdescribed above may be implemented utilizing hardware, a processorexecuting firmware instructions, a processor executing softwareinstructions, or any combination thereof. When implemented utilizing aprocessor executing software or firmware instructions, the software orfirmware instructions may be stored in any computer readable memory suchas on a magnetic disk, an optical disk, or other storage medium, in aRAM or ROM or flash memory, processor, hard disk drive, optical diskdrive, tape drive, etc. The software or firmware instructions mayinclude machine readable instructions that, when executed by one or moreprocessors, cause the one or more processors to perform various acts.

When implemented in hardware, the hardware may comprise one or more ofdiscrete components, an integrated circuit, an application-specificintegrated circuit (ASIC), a programmable logic device (PLD), etc.

While the present invention has been described with reference tospecific examples, which are intended to be illustrative only and not tobe limiting of the invention, changes, additions and/or deletions may bemade to the disclosed embodiments without departing from the scope ofthe invention.

What is claimed is:
 1. A method, comprising: transmitting, by a firstcommunication device, a first physical layer protocol data units (PPDU)that includes a first null data packet announcement (NDPA) frame as partof a first ranging measurement exchange, wherein the first rangingmeasurement exchange is among a set of ranging measurement exchanges;transmitting, by the first communication device, a first null datapacket (NDP) as part of the first ranging measurement exchange;recording, by the first communication device, a transmit time of thefirst NDP; determining, by the first communication device, whether asecond NDP was received correctly from a second communication device aspart of the first ranging measurement exchange; and determining whethera second NDP was received correctly includes determining, by the firstcommunication device, whether the first communication device beganreceiving the second NDP within an expected time period.
 2. The methodof claim 1, wherein: determining whether the second NDP was receivedcorrectly includes determining, by the first communication device,whether the first communication device began receiving the second NDPwithin an expected time period; the method further comprises: inresponse to determining that the first communication device did notbegin receiving the second NDP within the expected time period,performing, by the first communication device, a backoff operation; andthe second ranging measurement exchange is commenced after the backoffoperation.
 3. The method of claim 1, wherein: determining whether thesecond NDP was received correctly includes determining, by the firstcommunication device, whether the first communication device beganreceiving the second NDP within an expected time period; the methodfurther comprises: in response to determining that the firstcommunication device did not begin receiving the second NDP within theexpected time period, determining, by the first communication device,whether a channel medium is idle for a predetermined time period; andthe second ranging measurement exchange is commenced in response todetermining that the channel medium is idle for the predetermined timeperiod.
 4. The method of claim 1, wherein: determining whether thesecond NDP was received correctly includes determining, by the firstcommunication device, whether the first communication device correctlydecoded one or more signal fields in the second NDP; the method furthercomprises: in response to determining that the first communicationdevice did not correctly decode the one or more signal fields in thesecond NDP, performing, by the first communication device, a backoffoperation; and the second ranging measurement exchange is commencedafter the backoff operation.
 5. The method of claim 1, wherein:determining whether the second NDP was received correctly includesdetermining, by the first communication device, whether the firstcommunication device correctly decoded one or more signal fields in thesecond NDP; the method further comprises: in response to determiningthat the first communication device did not correctly decode the one ormore signal fields in the second NDP, determining, by the firstcommunication device, whether a channel medium is idle for apredetermined time period; and the second ranging measurement exchangeis commenced in response to determining that the channel medium is idlefor the predetermined time period.
 6. The method of claim 1, furthercomprising: receiving, by the first communication device, the second NDPcorrectly; recording, by the first communication device, a receive timeof the second NDP; determining, by the first communication device,whether a third PPDU was received correctly as part of the first rangingmeasurement exchange, wherein the third PPDU includes rangingmeasurement feedback from the second communication device; and inresponse to determining that the third PPDU was not received correctly,commencing, by the first communication device, the second rangingmeasurement exchange, including transmitting the second PPDU thatincludes the second NDPA frame as part of the second ranging measurementexchange.
 7. The method of claim 6, further comprising: in response todetermining that the third PPDU was not received correctly, performing,by the first communication device, a backoff operation; and the secondranging measurement exchange is commenced after the backoff operation.8. The method of claim 6, further comprising: in response to that thethird PPDU was not received correctly, determining, by the firstcommunication device, whether a channel medium is idle for apredetermined time period; and the second ranging measurement exchangeis commenced in response to determining that the channel medium is idlefor the predetermined time period.
 9. The method of claim 6, furthercomprising: in response to determining that the third PPDU was receivedcorrectly, commencing, by the first communication device, the secondranging measurement exchange a predetermined time period after an end ofreception of the third PPDU without determining whether a channel mediumis idle.
 10. The method of claim 1, further comprising: calculating, bythe first communication device, a distance between the firstcommunication device and the second communication device, usingmeasurements corresponding to the set of ranging measurement exchanges.11. An apparatus, comprising: a network interface device associated witha first communication device, wherein the network interface devicecomprises one or more integrated circuit (IC) devices configured to:transmit a first physical layer protocol data units (PPDU) that includesa first null data packet announcement (NDPA) frame as part of a firstranging measurement exchange, wherein the first ranging measurementexchange is among a set of ranging measurement exchanges, transmit afirst null data packet (NDP) as part of the first ranging measurementexchange, record a transmit time of the first NDP, determine whether asecond NDP was received correctly from a second communication device bythe first communication device as part of the first ranging measurementexchange including whether the first communication device beganreceiving the second NDP within an expected time period.
 12. Theapparatus of claim 11, wherein the one or more IC devices are configuredto: as part of determining whether the second NDP was receivedcorrectly, determine whether the first communication device beganreceiving the second NDP within an expected time period; in response todetermining that the first communication device did not begin receivingthe second NDP within the expected time period, perform a backoffoperation.
 13. The apparatus of claim 11, wherein the one or more ICdevices are configured to: as part of determining whether the second NDPwas received correctly, determine whether the first communication devicebegan receiving the second NDP within an expected time period; inresponse to determining that the first communication device did notbegin receiving the second NDP within the expected time period,determine whether a channel medium is idle for a predetermined timeperiod; and commence the second ranging measurement exchange in responseto determining that the channel medium is idle for the predeterminedtime period.
 14. The apparatus of claim 11, wherein the one or more ICdevices are configured to: as part of determining whether the second NDPwas received correctly, determine whether the first communication devicecorrectly decoded one or more signal fields in the second NDP; inresponse to determining that the first communication device did notcorrectly decode the one or more signal fields in the second NDP,perform backoff operation; and commence the second ranging measurementexchange after the backoff operation.
 15. The apparatus of claim 11,wherein the one or more IC devices are configured to: as part ofdetermining whether the second NDP was received correctly, determinewhether the first communication device correctly decoded one or moresignal fields in the second NDP; in response to determining that thefirst communication device did not correctly decode the one or moresignal fields in the second NDP, determine whether a channel medium isidle for a predetermined time period; and commence the second rangingmeasurement exchange in response to determining that the channel mediumis idle for the predetermined time period.
 16. The apparatus of claim11, wherein the one or more IC devices are configured to: record areceive time of the second NDP; determine a third PPDU was receivedcorrectly as part of the first ranging measurement exchange, wherein thethird PPDU includes ranging measurement feedback from the secondcommunication device; and in response to determining that the third PPDUwas not received correctly, commence the second ranging measurementexchange, including transmitting the second PPDU that includes thesecond NDPA frame as part of the second ranging measurement exchange.17. The apparatus of claim 15, wherein the one or more IC devices areconfigured to: in response to determining that the third PPDU was notreceived correctly, perform a backoff operation; and commence the secondranging measurement exchange after the backoff operation.
 18. Theapparatus of claim 15, wherein the one or more IC devices are configuredto: in response to that the third PPDU was not received correctly,determine whether a channel medium is idle for a predetermined timeperiod; and commence the second ranging measurement exchange in responseto determining that the channel medium is idle for the predeterminedtime period.
 19. The apparatus of claim 15, wherein the one or more ICdevices are configured to: in response to determining that the thirdPPDU was received correctly, commence the second ranging measurementexchange a predetermined time period after an end of reception of thethird PPDU without determining whether a channel medium is idle.
 20. Theapparatus of claim 11, wherein the one or more IC devices are configuredto: calculate a distance between the first communication device and thesecond communication device, using measurements corresponding to the setof ranging measurement exchanges.